A technique of using a near-field light has been suggested as a technique to record or reproduce data on or from an optical disk at higher density.
As condensing means for generating a near-field light, an optical system combining a condensing lens and a solid immersion lens (hereafter also called “SIL”) is attracting attention lately. If a condensing lens and an SIL are combined, the condensing means can implement a higher numerical aperture than a numerical aperture of the condensing lens. Since increasing the numerical aperture of the condensing means makes it possible to decrease the size of the spot, higher density recording becomes possible.
In the case of an optical system using an SIL, light emitted from the emitting surface of the SIL must enter the surface of the optical disk, therefore it is demanded that the distance between the SIL and the surface of the optical disk is extremely short. In the case of an optical system used for reproducing information from a DVD or the like, the distance between the objective lens and the surface of the optical disk is about 1 mm. Whereas in the case of an optical system using an SIL, the distance between the emitting surface of the SIL and the surface of the optical disk (hereafter also called “gap”) must be controlled to be at most several tens nm. In order to implement this control, a method called “gap servo” (or gap control) has been suggested. Gap servo is disclosed in Patent Literature 1.
Further, in the case of the optical system using the SIL, the distance between the emitting surface of the SIL and the surface of the optical disk is short, as mentioned above. Therefore if the emitting surface of the SIL and the surface of the optical disk are relatively inclined toward each other, the SIL may collide with the optical disk. The tolerance of this inclination is much smaller than the case of a DVD or the like. The inclination (hereafter also called “tilt”) could be generated by a warp generated when manufacturing the optical disk, or by an error generated upon assembling the optical system. Therefore not only the gap servo, but also a control to make the emitting surface of the SIL and the surface of the optical disk parallel with each other (this is called “tilt servo” or “tilt control”) is required in order to reproduce information using the SIL.
A method for operating the tilt servo using the near-field light is disclosed in Patent Literature 2, for example. In the case of the method disclosed in Patent Literature 2, if the emitting surface of the SIL and the surface of the optical disk are not parallel, in the SIL emitting surface the distance between the emitting surface and the surface of the disk becomes inconstant, and distribution of light quantity of the spot of the returned light reflected on the emitting surface of the SIL becomes uneven. Using this phenomena, the return light spot is detected by a divided detector, and a signal indicating the tilt between the emitting surface of the SIL and the surface of the optical disk is obtained, so as to control the inclination of the condensing means based on this signal.
According to the method disclosed in Patent Literature 2, if the condensing means shifts in the radius direction due to decentering of the optical disk, the position of the return light spot shifts on the detector, and an error is generated in the signal to be obtained. A method for avoiding this problem is disclosed in Patent Literature 3, for example. According to the method disclosed in Patent Literature 3, the form of the detector is divided so that the quantity of light that enters the detector for inclination detection does not change, even if the spot shifts in the radius direction in a state where the intensity of the annular area of the return light spot is high.
In the above mentioned conventional method, however, the following problems exist.
In order to reproduce information at high quality in the optical system using the SIL, the gap must be minimized so that optical transmission efficiency between the SIL and the optical disk is increased. The intensity of the return light spot in the annular area depends on the intensity of the beam that satisfies n·sin β>1 (n is a refractive index of the lens medium, and β is an incident angle of the beam) reflecting on the surface of the SIL. As the gap becomes smaller, a component of the beam that satisfies n·sin β>1 is transmitted more to the optical disk, so the intensity of the annular area decreases, and the difference of the intensities between the annular area and the inner side of the annular area decreases. As a result, even if the detector is divided using the method disclosed in Patent Literature 3, the quantity of light that enters the detector for inclination detection changes if the spot shifts in the radius direction in a state where gap servo is operated with a small gap.